The present invention relates to a film packaging machine and a film packaging method for automatically packaging a to-be-packaged object, such as a flat tray containing foodstuff, by using a stretchable film or the like, and a film gripper for holding the film in the automatic packaging operation.
There are film packaging machines that can automatically package a to-be-packaged object together with a tray in a stretchable packaging film of polyvinylchloride resin. These machines are described in, for example, Australian Patents Nos. AU-A-59313/94 and AU-A-57640/94. One such film packaging machine comprises a movable front gripper and a pair of movable side grippers. The front gripper is used to seize a delivery-side end portion of the film and draw out the film toward the object to the packaged. The side grippers, which are arranged on either side of the drawn-out film with respect to the width direction thereof, serve individually to seize the opposite side edge portions of the film and extend the film width.
In the film packaging machine, the film drawn out by means of the front gripper is stretched in the width direction by means of the side grippers, and is put on a tray that contains a foodstuff therein. Thereafter, the film is tucked under the outer surface of the bottom of the tray from both sides in the delivery direction (drawing direction of the front gripper) and in the width direction, whereupon the tray is packaged.
These individual grippers constitute a so-called film gripper, and each include an upper clamper in the form of a flat iron plate or the like and an arm-shaped Flower clamper that is disposed under the upper clamper so as to move open-and-close directions relative to the upper clamper. Polyurethane sponge rubber with a closed-cell structure is put on the upper surface of the distal end portion of the lower clamper. This sponge rubber is caused to touch and leave the lower surface (film contact surface) of the distal end portion of the upper clamper when the lower clamper is moved open-and-close directions. Each gripper is moved toward a side edge portion or other part of the film with its lower clamper rotate downward so that the two clampers are open. When the lower clamper is moved upward after the side edge portion or the like is interposed between the upper and lower clampers and clamped by the clampers, the clampers are moved toward their original position. In this manner, the film is drawn out or the drawn-out film is stretched in its width direction.
During this stretching operation, the film is kept gripped without slipping off the gripper as the aforesaid rubber (polyurethane sponge rubber) is in contact with the film.
Besides the film packaging machine constructed in this manner, there is a known film packaging machine of an alternative construction. In this second type, one of a pair of side grippers that are located on either side of a film is immovable, while the other is movable, in the case of the drawn-out film is stretched in its width direction. The film is stretched in its width direction as the other side gripper moves. The grippers of this packaging machine, like the aforementioned ones, use the closed-cell polyurethane sponge rubber for their film contact members, whereby the film can be kept gripped.
The assignee hereof has recently proposed a novel film packaging machine. In this packaging machine, film contact members of a film gripper are formed of silicone rubber in place of the conventional closed-cell polyurethane sponge rubber, whereby reduction of the film gripping force can be restrained even when the machine is operated at high humidity.
Film packaging machines may possibly be used in various environmental conditions including highly humid working conditions, regions, and times, e.g., on watered floors of kitchens. In order to achieve satisfactory automatic packaging operation without regard to the working conditions, therefore, the film holding performance of each gripper must be maintained to prevent a stretched film from unexpectedly slipping off the gripper.
However, this problem cannot be solved by the conventional film packaging machines that use the closed-cell polyurethane sponge rubber for their film contact members.
If the film or grippers sweat during use at high humidity or due to changes in temperature in the working environment, therefore, water makes the film held by the grippers so slippery on the aforesaid rubber that the possibility of the film slipping off the grippers increases, thus resulting in defective packaging.
A result represented by curve A in FIG. 13 (mentioned later) was obtained from a frictional force measurement test conducted by the inventor hereof.
The following test conditions were employed. In FIG. 11 showing a tester, numeral 201 denotes a base plate; 202, a top plate mounted on the base plate 201 by means of a stud 203; and 204, film contact members fixed to the upper surface of the base plate 201 and the lower surface of the top plate 202 by means of double-side-coated adhesive tapes 205, individually. Further, numerals 206, 207 and 208 denote a weight, a film specimen (sample) 40 mm wide and 25 mm long, and a tension gage anchored to a sheet metal 209 that is bonded to one end portion of the specimen 207.
In the frictional force measurement test, the specimen 207 is interposed between the upper and lower film contact members 204, and the weight 206 of 200 grams is placed on the top plate 202. In this state, the tension gage 208 is pulled in the horizontal direction, and the resulting tensile load or frictional force is read. The frictional force is measured at 25.degree. C. by means of the tension gage 208 with the humidity gradually increased from 40% by 10% at a time.
The result of this frictional force measurement test indicates that the gripper using the conventional closed-cell polyurethane sponge rubber for its film contact members exhibits a relatively small frictional force even at low humidity and its film gripping force or frictional force decreases as the humidity increases. As is evident from this result, the conventional polyurethane sponge rubber has good film-releasability, and actually it is known that the rubber has a good releasability.
Although there are no obvious reasons why the film gripping force decreases in the aforesaid manner, the following phenomena may possibly be the cause of this effect. A chloroethylene film, which is conventionally used for packaging, contains a cloud preventive such as a surfactant. It is supposed that the cloud preventive oozes out and adheres to the respective film contact surfaces of the film contact members of polyurethane sponge rubber, thereby changing conditions for contact with the film (or making the film slippery), while a to-be-packaged object is being packaged. Since water on the film never moves once it gets into a space between the film contact surfaces of the closed-cell polyurethane sponge rubber, a water film is inevitably formed between the film and the rubber surfaces.
Thus, as mentioned before, the film packaging machine using the conventional film gripper and the film packaging method carried out by means of this machine are subject to the problem that the gripped film becomes liable to slip off, thereby causing defective packaging, as the humidity increases. It was ascertained that a small frictional force cannot prevent the film from slipping off at the humidity of 90%, as indicated by curve A in FIG. 13.
In FIG. 13, curve G represents a result of the aforesaid frictional force measurement test conducted in the same conditions on silicone rubber for film contact members. As seen from this result, the silicone rubber film contact members have a greater low-humidity frictional force than the conventional ones that are formed of polyurethane sponge rubber. While the frictional force slightly increases at the humidity of 60% and below, it decreases as the humidity increases thereafter. Nevertheless, the silicone rubber contact members can maintain a frictional force much greater than that of the conventional polyurethane sponge rubber. Thus, a film gripper using this silicone rubber is not subject to any substantial reduction in film gripping force.
The inventor hereof ascertained, however, that the film gripper based on the silicone rubber is poor in film-releasability (or capability in separating from the gripped film). Although the cause of this drawback has not yet been cleared up, it may possibly be attributed to the following inclinations of the gripper. The surface of a silicone rubber contact member may be made apparently soft and sticky by pressure (gripping force) applied thereto during use, chemical change attributable to wear, and oozes of siloxane or other low-molecular materials that are contained in a plasticizer in the silicone rubber. Otherwise, the silicon rubber surface may be smoothed down by abrasion, so that the film can more easily adhere to the rubber, thus increasing frictional resistance.
If the film-releasability is low, as described above, left- and right-hand film grippers sometimes may be ill balanced as they release the film or may drag the film when they are opened and return to their respective original positions after the film is tucked under the outer surface of the base of a to-be-packaged object. Accordingly, lap portions of the film on the underside of the object may not be long enough, the film may be torn, or the object may be dragged together with the film, thus resulting in defective packaging.